Method of making articles from aluminous metal powder



United States Paten METHOD OF MAKING ARTICLES FROM ALUMINOUS POWDER Kensi igton, Pa., assignors to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania Application October 12, 1954, Serial No. 461,904

"7 Claims. (Cl. 75-226) No Drawing.

This invention relates to making wrought products from compacted bodies of aluminous metal powder, the products thus made having properties superior to corresponding articles fabricated from cast metal. aluminous metal as here used refers to bothv aluminum and" the alloys in which this element predominates. It also. is intended to cover mixtures of powdered metals in which aluminum or aluminumlbase alloy powder is the major component.

Greatadvances have been made in recent years in the number and variety of products fabricated from nonaluminous metal powders, especially iron powder. The products ofier advantagesnot found in similar articles made from the solid. metal. The ,powder products have beenof two types, one in which-the powder is pressed into the finaishape and the second one in which a block or other simple shape is initially producedthat is subsequently worked or molded to final form. Progress in the field of aluminous metal powder products has been slow, among the reasons for this being the difliculty of securing a good bond between themetal particles and the adherence of the powder to the walls of the forming die. Attempts to utilize the practices employed in handling other metal powders have often failed when applied to aluminous powder .or;have resulted in costly or unsatisfactory products.

This invention is directed to overcoming prior difiiculties and providing a commercially useful process of making worked aluminous metal powder products. A particular object of this invention is to provide a method of preliminarily compacting, without the use of high pressures, aluminous metal powder bodies which are adapted to being worked by conventional processes. An-

other object is to provide a method of making aluminous metal powder compacts suitable for extrusion. Still another object is to-provide a simple and effective method of making compacted aluminous metal powder bodies in large quantities that can'be easily handled and used for the fabrication of wrought products. A further object' is to provide wrought aluminous metal powder products having better properties, especially at elevated temperatures, than similar productsmade from cast metal.

Our invention is predicated upon the discovery that worked aluminous metal powder products can be easily fabricated from p owder compacts which have been formed under a relatively low pressure of less than 1,000 lbs./ sq. in. while heating the powder mass to a temperature between 840" F. and the liquidus temperature of the alloy or the melting point of aluminum in case an alloy is not used, for a sufficient length of time to produce a density of at least 30% of the solid metal. At the end of the heating period, the compacted mass may be cooled to room temperature, or it may be held at an elevated temperature preparatory to hot. working or it may be directly transferred to apparatus for hot working; If the compact has beencooledto room temperature it should be re-heated forany hot working. A protective atmos- The term the powder is not exposed to the furnace atmosphere. On the other hand it may be desirable to use a. protective atmosphere if the powder is not covered or is in contact with the furnace atmosphere.

The aluminous metalpowders which are adapted to be compacted and hotworked by our process may be of the flake or of the atomized type. A wide variation in particle size is permissible but the particles should not be larger than will pass through a mesh screen. However, to produce wrought articles having the highest strength values, it is desirable to utilize powders of a very fine mesh size, such as those passingthrough a 325 mesh screen. Under some circumstances it is advisable to employ powders of difierent sizes and to thoroughly mix them before compacting the mass. The choice of the form of powder and the proportion of different sizes isa matter of determination from tests in individual cases and is within the skill of those familiar with the powder art. a a

The metal powder used in our process may consist of aluminum of low purity, for example 99%, up to the biggest purity obtainable, or it may consist of particles of aluminum base alloys, or it may be composed of a mixture of particles of aluminum and the desired alloying elements. The elements which may be thus associated with aluminum in either alloyed or elemental form are those commonly employed in the aluminum base alloy art, such as copper, magnesium, silicon, zinc, manganese, iron and certain high melting point elements. They may be present in the quantity normally used. in the solid metal alloys, for example, up to 10% copper,

- up to 15% silicon, up to 10% zinc, up to 10% magphere is notrequired; during compacting and heating if nesium. Such high melting point elements as titanium, boron, and zirconium may also be employed in amounts up to 0.5%. It may be necessary or desirable to subject "the alloyed compact or subsequent product to a specific thermal treatment to secure diffusion of the'alloying elements throughout the aluminum matrix with or without a subsequent precipitationhardening treatment. Such thermal treatments are known in the art and do not form a part of our process.

In addition to the aluminum and the alloying elements there is a certain amount of aluminum oxide present in the powder and products made therefrom. A natural .oxide coating forms on the aluminous metal particles upon exposure to air because of the oxidizable character of aluminum. The quantity of oxide perunit weight of metal becomes larger as the size of the particle decreases.

The presence of the oxide has been found to be advantageous, especially where the product is to be exposed to service at elevated temperatures. It is even desirable at times to promote oxidation during preparation of the powder to increase its oxide content and thus improve certain properties of the compacted products. The amount of oxide present may vary over a wide range depending upon the properties desired but usually it should bebetween about 0.1 and 30% by weight of the powder.

To prepare the initial compact a charge of powder should be introduced toa chamber or die which is closed by means of a piston or other slidable closely fitting member. A convenient arrangement where high production is desired is to provide a series of containers which can be separately filled at room temperature and pressure and a piston or other closure device fitting the open end. An appropriate low pressure may then be applied to the piston or closure before the container enters the heating chamber or the pressure may be applied after the closed container has been introduced into the furnace. In some cases it is sufiicient to place a weight upon the piston cient mass itself to provide the necessary weight and pressure on the powder charge. An alternative method of applying pressure is to use a power or spring loaded piston. Since the pressure applied is of such a low order, it can be exerted by simple means and thus obviate the need for special presses and high pressure equipment. This represents a considerable economy in the production of powder compacts.

The magnitude of the pressure necessary to consolidate the powder mass is relatively low, far less than that required for extrusion or that had previously been considered possible in the aluminous powder metal art. It has been determined that pressures as low as only a few ounces up to 1,000 lbs/sq. in. are adequate to produce the necessary compaction of the powder within the temperature range herein defined. Preferably the pressure should be within the range of 5 to 800 lbs./ sq. in.

While under the foregoing pressure, the charge of powder should be heated to a temperature which insures annealing, metal flow and a partial welding of the metal particles. Temperatures between 840 F. and the liquidus temperature of the alloy or the melting point of aluminum (which is generally considered to be 1220 F.) have been found to produce this result. Generally speaking, if the temperature is in the lower portion of the range, a longer period of heating and a higher pressure are required. For the best results, however, the temperature should exceed 1000 F.

The heating of the powder under pressure should extend for a sufficient time to effect consolidation to the point where the compact achieves a density of at least 30% that of of the solid metal. A higher density is usually preferred, on the order of 50% or more to insure integrity of the compact. Generally, the desired compaction can be accomplished in a shorter time within the upper portion of the temperature range and at pressures above 100 lbs/sq. in. Also, the size of the compact must be taken into consideration in selecting a suitable time and temperature. In terms of a definite time the heating period may extend from A to 36 hours.

When the powder compact has attained the desired density, the heating is terminated and the compact may be cooled to a much lower temperature or it may be maintained at a high temperature and then hot Worked. The pressure may be continued or released during the cooling period. If the compact is cooled to room temperature it may be scalped to remove any surface defects and then reheated to the hot working temperature. A compact made according to our process has a sufficient firmness to withstand such handling and machining operations. If, instead of being cooled, the compact is to be hot worked upon removal from the container, it may be transferred to an extrusion or forging press or other hot working means and reduced to the desired shape. In some instances it may be desirable to place the hot compact in a soaking furnace before performing the hot work.

It has been found that the initial aluminous metal compacts produced by the foregoing low pressure method have a suflicient density and cohesiveness to withstand the customary handling operations incident to scalping, heating, extrusion, and forging. The aluminous metal particles become at least partially bonded to each other during the low pressure compacting operation and that further bonding develops with subsequent hot working.

For hot working, the compact should preferably be at a temperature between 600 and 1160 F., the precise temperature chosen being determined, in part, by the nature of the operation and the composition of the compact. For example, to forge a compact, the tempera ture should be between 700 and 800 F. On the other hand for extrusion, the temperature range may be 700 to 1160 F. The usual aluminum hot working practices and equipment may be employed in fabricating the compacts which is obviously advantageous.

A charge of flake type aluminum powder consisting of 98% minus 325 mesh particles of 99.5% purity aluminum, exclusive of oxide, was introduced to a 6 /2 inch 1. D. x 48 inch long graphite tube which filled the tube to a depth of 36 inches. A close fitting disc of graphite was put on top of the powder charge and a weight placed on the disc which created a pressure of 54 lbs./ sq. in. on

the powder. The tube with charge was placed in an air atmosphere furnace in an upright position and heated to 1175 F. for /2 hour. The tube and contents were then cooled to room temperature in the furnace and the powder compact ejected from the tube. The compact thus formed was found to have a density of 1.6 g./cc., or 59% of that of the solid metal. The compact was scalped, re-heated to 900 F. and extruded into inch diameter rod having a dense uniform structure. Test specimens cut from the rod had, at room temperature, a tensile strength of 36,000 lbs./sq. in., a yield strength of 24,000 lbs/sq. in. and an elongation of 15%. Other specimens heated to 600 F. for 1000 hours and tested at that temperature, had a tensile strength of 16,000 lbs/sq. in., the yield strength was 15,000 lbs/sq. in., and the elongation was 17%. In comparison, aluminum sheet of 99.2% purity in the hardest temper, known as H 18, has at room temperature the following typical properties: tensile strength, 24,000 lbs./sq. in.; yield strength, 22,000 lbs./sq. in.; and elongation, 5%. At 600 F. the typical properties of the same material are: tensile strength, 2,500 lbs/sq. in.; yield strength, 1,500 lbs./ sq. in. and elongation A charge of atomized aluminum powder of 99.5% purity metal and consisting of minus 325 mesh particles was placed in a graphite tube of the same dimensions as in Example I which was filled to a depth of 36 inches. A graphite disc was fitted into the cylinder and 54 lbs/sq. in. pressure applied to the powder in the same manner as in Example I. The charged tube was heated in an upright position in an air atmosphere furnace to a temperature of 1180 F. and held at that temperature for A1 hour. Upon cooling to room temperature in the furnace the compact was ejected from the tube and found to have a density of 1.2 g./cc. or 45% of that of solid metal. The compact was scalped, re-heated to 900 F. and extruded into rod form. An examination of the rod showed it to have a dense uniform structure. At room temperature specimens cut from the rod where found to have a tensile strength of 27,000 lbs/sq. in., a yield strength of 17,000 lbs/sq. in. and an elongation of 25%. After being heated at 600 F. for 100 hours other specimens of the rod were tested at that temperature and found to have a tensile strength of 12,000 lbs/sq. in., a yield strength of 10,000 lbs/sq. in. and an elongation of 18%.

Having thus described our invention and certain embodiments thereof, we claim:

1. The method of making a wrought product from aluminous metal powder of less than 35 mesh size comprising initially forming a partially consolidated compact of said powder by compressing a loose mass of said powder under a pressure of less than 1,000 lbs/sq. in. at a temperature between 840 F. and the temperature at which the metal becomes completely molten until the compressed mass attains a density of at least 30% of that of the solid metal, and thereafter working said compact into the finished shape.

2. The method of making a hot worked product from aluminous metal powder of less than 35 mesh size comprising initially forming a partially consolidated compact of said powder by compressing a loose mass of said powder under a pressure of less than 1,000 lbs./sq. in. at a temperature between 840 F. and the temperature at which the metal becomes completely molten until the compressed mass attains a density at least 30% that of the solid metal, and thereafter hot working said compact at a temperature between 600 and 1160 F.

3. The method of making a wrought product from aluminous metal powder of less than 35 mesh size comprising initially forming a partially consolidated compact of said powder by compressing a loose mass of said powder under a pressure of to 800 lbs./sq. in. at a temperature between 1000 F. and the temperature at which the metal becomes completely molten until the compressed mass attains a density of at least 50% of that of the solid metal and thereafter Working said compact into the finished shape. a

4. The method of making a hot worked product from aluminous metal powder of less than 35 mesh size comprising initially forming a partially consolidated compact of said powder by compressing a loose mass of said powder under a pressure of 5 to 800 lbs./sq. in. at a temperature between 1000 F. and the temperature at which the metal becomes completely molten until the compressed mass attains a density of at least 50% of that of the solid metal and thereafter hot working said compact at a temperature between 600 and 1160 F.

5. The method of making a hot worked product from aluminous metal powder of less than 35 mesh size comprising initially forming a partially consolidated compact of said powder by compressing a loose mass of said powder under a pressure of less than, 1,000 lbs./sq. in.

at a temperature between 1000 F. and the temperature at which the metal becomes completely molten until the compressed mass has attained a density of at least 30% of that of the solid metal, cooling said compact to room temperature, re-heating it to 600 to 1160 F., hot working it and thereafter cooling the worked article to room temperature.

- 6. The method of making a hot extruded product from aluminous metal powder of less than 35 mesh size comprising initially forming a partially consolidated compact of said powder by compressing a loose mass of said powder under a pressure of 5 to 800 lbs./ sq. in. at a temperature between 1000 F. and the temperature at which the metal becomes completely molten until the mass attains a density of at least 30% of that of the solid metal, cooling the compact to room temperature, scalping, reheating to between 700 F. and 1160 F. and thereafter extruding said compact. V

7. The method of making a hot forged product from aluminous metal powder of less than 35 mesh size comprising initially forming a partially consolidated compact of said powder by compressing a loose mass of said powder under a pressure of 5 to 800 lbs/sq. in. at a temperature between 1000" F. and the temperature at which the metal becomes completely molten until the mass attains a density of at least of that of the solid metal, and thereafter forging said compact at 700 to 800 F.

References Cited in the file of this patent UNITED STATES PATENTS 2,290,734 Brassert July 21, 1942 2,287,251 Jones June 23, 1942 2,391,752 Stern Dec. 25, 1945 FOREIGN PATENTS 573,113 Great Britain Nov. 7, 1945 625,364 Great Britain June 27, 1949 706,528 Great Britain Mar. 31, 1954 706,536 

1. THE MOTHED OF MAKING A WROUGHT PRODUCT FROM ALUMINOUS METAL POWDER OF LESS THAN 35 MESH SIZE COMPRISING INTIALLY FORMING A PARTIALLY CONSOLIDATION COMPACT OF SAID POWDER BY COMPRESSING A LOOSE MASS OF SAID POWDER UNDER A PRESSURE OF LESS THAN 1,000LBS./SQ.IN. AT A TEMPERATURE BETWEEN 840*F. AND THE TEMPERATURE AT WHICH THE METAL BECOMES COMPLETELY MOLTEN UNTIL THE COMPRESSED MASS ATTAINS A DENSITY OF AT LEAST 30% OF THAT OF THE SOLID METAL, AND THEREAFTER WORKING SAID COMPACT INTO THE FINISHED SHAPED. 